1. Field of the Invention
The present invention relates to a backlight device and a display device, and more particularly, to a display device including a display panel for displaying a video image through adjustment of a transmittance of light from a light source and a backlight for illuminating the display panel.
2. Description of the Related Art
In recent years, the thickness of a display such as a display device using liquid crystal has been greatly reduced. A liquid crystal display device mainly includes three modules. A first one of the modules includes a liquid crystal display panel module in which liquid crystal is enclosed between two glass substrates, and a voltage is supplied to the liquid crystal to change the orientation of the liquid crystal, to thereby change the light transmittance (modulation degree of light passing through the liquid crystal). In the liquid crystal display panel module, liquid crystal cells constituting pixels are arranged two-dimensionally, and the liquid crystal cells are sequentially controlled, to thereby attain two-dimensional control of the light transmittance. A second one of the modules includes a backlight module, which is provided in the rear of the liquid crystal display panel module so as to be used as a light source and emits illumination light. The illumination light is supplied from the rear of the liquid crystal display panel module, and hence the liquid crystal display device produces displays. A third one of the modules includes a control part, which is a module for controlling the liquid crystal display panel module and the backlight module.
Conventionally, a cold cathode fluorescent lamp (CCFL) has been widely used as a light source for the backlight of a liquid crystal panel. In recent years, however, a light emitting diode (LED) instead of the CCFL is also used as the light source for the backlight. It is easy to perform on-off control on the light emitting period of the LED, and the light emission amount thereof is also easy to control through control of a current amount. Hence, the LED is lower in power consumption as compared with the CCFL. Further, the LED is physically smaller in configuration than the CCFL, which may reduce an area for the light source for the backlight formed of one LED device.
As a technology of using a smaller light source area, JP 2001-142409 A discloses a technology which relates to an area light modulation technology using an LED. According to the area light modulation technology, LEDs which serve as light sources to emit illumination light for irradiating a liquid crystal panel are provided at least one by one for a plurality of divided areas, and an LED control circuit controls, in accordance with a video signal, driving of LEDs on a backlight panel in the unit of the divided areas so that only picture areas requiring illumination light may be at least irradiated with illumination light, without irradiating with the illumination light picture areas which do not need to be illuminated basically, to thereby reduce power consumption necessary for illumination.
There is a connection method for use in a case of using LEDs as the backlight, as disclosed in JP 2006-352011A, in which a constant current source and a switch are connected in series for one LED chain, and a constant current is caused to flow through the LED chain by turning on and off the switch, to thereby turns on and off the LEDs.
Further, JP 2002-182182 A and JP 2007-123233 A disclose a scanning backlight (backlight scrolling) driving technology. According to the scanning backlight driving technology, a screen is divided into a plurality of areas in a longitudinal direction, and the backlight is sequentially driven at predetermined intervals, to thereby produce an effect of reducing a motion blur.
The area light modulation technology disclosed in JP 2001-142409 A may be implemented with relative ease in a backlight which has LEDs arranged in a grid pattern. Further, in order to implement the combination of the scanning backlight driving technology disclosed in JP 2002-182182 A and JP 2007-123233 A and the area light modulation technology, the light sources for the backlight are distributed to be arranged in a grid pattern in the unit of a plurality of areas, and LEDs connected to one area are controlled as one LED chain, to thereby allow simultaneous control of the area light modulation and the scanning backlight driving.
However, in a case where the light sources for the backlight such as LEDs are distributed to be arranged for a plurality of areas, each of the divided areas requires an LED chain as a dedicated control chain for area control, and when the number of divided areas increases, the control module such as a constant current source is increased in size while the number of control signals between the control module and the backlight module increases. Further, along with an increase in number of LED chains, it is necessary to provide a plurality of the constant current sources, which leads to a problem of a large heating value.